Diesel engines may be used to generate efficient combustion and conversion of fuel into work. However, efficient diesel combustion may create conditions that can be undesirable for an aftertreatment device. For example, while various types of combustion may improve engine efficiency and decrease concentrations of NOx and particulate emissions, it may also lead to lower exhaust temperatures, which may negatively impact the performance of various aftertreatment devices, particularly when a diesel engine is operating at a light load and/or at idle.
One approach to control the temperature of exhaust gas passing through an aftertreatment device in diesel engines is described in U.S. Application No. 2004/0060284 A1. In the '284 reference, intake and/or exhaust valve timing, either singly or in combination with fuel injection timing and selective individual cylinder cutout, are controlled in response to sensed engine operating parameters. The exhaust valve opening event allows energy previously available to the piston to be transported to the exhaust system in the form of thermal energy carried by the exhaust mass flow. Thus, exhaust temperature can be controlled by the intake valve and exhaust valve timing.
However, the inventors herein have recognized that operation according to '286 may increase NOx production while attempting to achieve an increased exhaust gas temperature optimal for aftertreatment devices. Furthermore, the inventor herein has recognized that some types of variable valve timing operation may degrade engine efficiency and increase pumping work. For example, although early exhaust opening increases the exhaust gas temperature, it may also entail early exhaust valve closing time, which can decrease engine efficiency by increasing pumping work. Thus, the approach of '286 may adjust intake valve timing or exhaust valve timing to increase the exhaust temperature while sacrificing engine efficiency and pumping work.
Another approach to diesel engine operation to achieve improved combustion is described in U.S. Pat. No. 6,405,694. This approach uses a control device that controls valve timing based on engine speed or load. In one embodiment, an exhaust valve advanced-closing control is executed so that the exhaust valve is closed at an earlier timing than the intake top dead center (TDC) and the intake valve is opened nearly at the intake TDC or at a later timing. This approach allegedly provides improved fuel atomization within the cylinder and reduced effects caused by deteriorated combustion.
However, the inventor herein has recognized that the control device in the '694 reference is independent of exhaust temperature concerns. As such, variation in exhaust gas temperature caused by said operation may cause the exhaust gas to become too hot or too cool for emission control devices.